1. Field
The invention is in the field of processes for removing metallic impurities from molten copper.
2. State of the Art
Refining of metallic copper from an impure state is accomplished conventionally by either fire-refining or electrolytic refining. Fire-refining is capable of removing substantially all the sulfur, zinc, tin, and iron impurities from copper, but bismuth, antimony, and arsenic impurities are only partially removed. At best, the latter impurities can be reduced to concentrations of only about 100 ppm (parts per million) in the molten copper by conventional fire-refining. It is possible to further reduce the level of these impurities by electrolytic refining. However, such additional processing is economically feasible only when the fire-refined copper contains other precious metals in sufficient amounts so that the recovery thereof offsets the expense of the electrolytic refining step.
Attempts have been made to develop an economical method of reducing the levels of bismuth, antimony, and arsenic during or in conjunction with the conventional steps employed in the fire-refining of impure copper. For example, it has been proposed to contact the molten copper resulting from conventional fire-refining techniques with a molten salt phase comprising copper fluoride, whereby residual metallic impurities in the molten copper phase are transferred to the salt phase. The molten salt phase can be brought into direct contact with the molten copper, or a material can be added to the molten copper which forms the salt phase in situ.
A process for removing gaseous and metallic impurities from copper base alloys is disclosed in U.S. Pat. No. 3,434,825 wherein a gaseous, monomeric, halogen-containing, lower aliphatic hydrocarbon having at least one fluoride atom, is bubbled through the molten copper base alloy. As the fluorine-containing hydrocarbon passes into the melt, it thermally decomposes and the fluorine preferentially reacts with impurities, such as aluminum, iron, chromium, etc. to form insoluble fluorides which separate from the melt by gravity. Unfortunately, the process of U.S. Pat. No. 3,434,825 is ineffective in removing bismuth, antimony, and arsenic from molten copper.